(1) Field of the Invention
It is an object of the present invention to provide a process for the production of coatings, wherein the sheet-like structures which can be produced therefrom have considerably improved properties in various respects. This includes the property that the lacquer which has been applied to a metallic substrate and which contains the copolymer solution in combination with organic polyisocyanates, shall, after drying in the air for three to four days at temperatures of about 20.degree. C., cure in such a way that the film no longer swells under the influence of water and also cannot be removed mechanically by scratching, even if the film is exposed to water for a brief period at temperatures of 50.degree.-70.degree. C.
It is a further object of the present invention to use in the reactive laquer such an acrylic resin, containing hydroxyl groups, which, in combination with polyisocyanates, provides the binder foundation for air-drying lacquers and also stoving lacquers which are suitable for top coats of lacquer in the automobile industry, the lacquer being present after stoving in layer thicknesses of about 40 to 80 .mu.m and the top lacquer being located directly on top of the stoved primer of electrocoating lacquer.
In the modern methods of the present time for lacquering motor vehicles in the automobile factory, the coachwork parts are usually provided, after phosphating, with a primer of electrocoating lacquer and the latter is stoved to give a plastic coating. A spraying lacquer or a coating composition, which is usually designated a filler, is then applied to this plastic coating, usually by a spray process, and is again stoved.
The pigmented formulation of automobile lacquer is then applied, in the desired pigmentation, to this two-layer coat of lacquer or primer, by spraying, and the automobile body is stoved again.
The complete coating, which, as explained above, consists of three layers, has a layer thickness of 70-100 .mu.m after stoving, of which the proportion relating to the outer layer of top lacquer is 30 to 40 .mu.m. In the course of methods of rationalisation in the automobile industry efforts are being made to change over from the three-layer system to a two-layer build-up. In this, the electrocoating lacquering or another equivalent primer is intended to form the first coating layer and the final lacquering with the pigmented lacquer is intended to be built up thereafter, the total thickness of the layers applied being intended, however, to have the same layer thickness. Using the conventional automobile top lacquers, however, it is not possible to produce, from pigmented lacquer formulations in the customary spraying and stoving treatment, coatings of a type which, after drying in the air or after stoving, give lacquer films which are completely uniform (that is to say free from so-called curtaining and free from craters and blisters).
In addition, the less viscous copolymer solutions used according to the invention exhibit a better absorption of pigment both when using inorganic pigments, such as, for example, titanium dioxide, or organic pigments, such as, for example, carbon black. When titanium dioxide is used as the pigment, the copolymers used according to the invention, and also the reactive lacquer manufactured therefrom, still produce high-gloss films at a pigment/binder ratio of 1.5 to 2:1, whilst the known copolymers and the known reactive lacquers manufactured therefrom exhibit a distinct falling off in gloss at the same level of pigmentation. In addition, it has been found that the copolymers used according to the invention and the reactive lacquers manufactured therefrom produce high-gloss lacquers with carbon black, whilst the known copolymers and the reactive lacquers manufactured therefrom display matt films at the same level of pigmentation.
(2) Prior Art
Numerous proposals have been disclosed to manufacture solvent-resistant and alkali-resistant lacquers by reacting polyisocyanates and copolymers containing hydroxyl groups and to convert them into coatings. DT-AS 1,247,006 describes a process for the production, by the polyisocyanate polyaddition process, of alkali-resistant sheet-like structures which are obtained from copolymers containing hydroxyl groups, and polyisocyanates, but which are not adequately water-resistant after a reaction time of three to four days at 20.degree. C. These coatings, obtained by this known process, are, therefore, not suitable as top lacquers for external lacquering which are resistant to weathering, since blistering takes place after a very short time and the adhesion of the lacquer to the various metallic substrates falls off and as a result, the lacquer comes away from the substrate.
It is also known that polyhydroxy compounds of polyols can be cured with polyisocyanates in order to obtain cross-linked lacquer coatings with good resistance properties. These combinations also display too low a resistance to water and a fairly low resistance to weathering of the cured lacquer films.
It cannot, however, be inferred from the state of the art that the choice of specified solvents and a specified quantity range of a combination of various polymerisable monomers and the use of a combination of initiators makes it possible to manufacture copolymer solutions which have a substantially higher solids content and which open up the use of the new copolymers as binders for lacquer systems which are based on acrylates and have a low solvent content.
It is found, surprisingly, that the copolymers which are used according to the invention and have a hydroxyl number of 130 to 200, preferably 140 to 165, give lower viscosities than the known copolymers which have the same hydroxyl numbers. Thus, in a 60% strength by weight solution in aromatic hydrocarbons/ethylglycol acetate, the copolymer solutions used according to the invention have viscosities of G to X, preferably H to U, measured by the Gardner-Holdt method at 20.degree. C. In this way the copolymer solutions used according to the invention take account of the demands of environmental protection for binders of high solids content and low solvent content.
Compared with the known reactive lacquer films based on copolymers and polyisocyanates, the films of the reactive lacquers used in the process of the invention have the following advantages: better resistance to solvents, higher abrasion resistance and better resistance to weathering in the Florida climate. In addition, the copolymers used according to the invention, when combined with polyisocyanates and diluted with acetone, xylene or butyl acetate to a viscosity of 25 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice, give lacquers of higher solids content which have a solids content of 50 to 65% by weight, preferably from 55 to 63% by weight, in the clear lacquer. When such clear lacquers or pigmented lacquers are applied to steel sheets by spray application and are exposed to the air for a brief period and subsequently stoved, high dry film layer thicknesses of 70 to 80 .mu.m are obtained, which do not exhibit crater-formation or blistering.